[racket] phases

Hello,

what's the incantation needed to make the following work:

========
#lang racket

(provide sofa-version)

(require ffi/unsafe)
(require ffi/unsafe/define)
(require racket/runtime-path)

(define sofa-version "20120301_a")

(define arch
  (if (fixnum? #e10e15) 'x64 'x86))

;; Access a file "data.txt" at run-time that is originally
;; located in the same directory as the module source file:
(define-runtime-path sofa.dll
  (format "sofa-~a-~a.dll" sofa-version arch))

(define-ffi-definer define-sofa (ffi-lib sofa.dll))
========

Please note that sofa-version is exported. I would like to not have to
write this constant string in two places.
(This is working if I copy manually the two constants in the format).

Also, since I am at it: putting the dll in the same directory as the
interface module or the executable is a satisfactory solution for my needs.
But if I want to distribute this package to others, what should I use as
argument to the ffi-lib function, to make everyone happy?

Thanks!
P.
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